1. Field of the Invention
This invention relates to a fixing device for use in an image forming apparatus and an image forming apparatus provided with the same.
2. Description of the Related Art
In electrophotographic image forming apparatus, such as copying machines, printers, and facsimile machines, a toner image formed on a photoconductive drum is transferred onto a recording sheet, and fixed onto the recording sheet by an application of heat and pressure to the recording sheet.
A fixing device used in such image forming apparatus includes a fixing roller and a pressing roller made in pressing contact with the fixing roller. Further, a rod-shaped heater is provided in the fixing roller, the pressing roller, or other heating member to heat the fixing roller to have a predetermined temperature. To melt toner particles adhered on a recording sheet under suitable conditions and accomplish good fixation, it is necessary to keep the fixing roller at a predetermined surface temperature. The surface temperature of the fixing roller is controlled by detecting a surface temperature of the fixing roller by a temperature sensor such as a thermistor directly or indirectly, and regulating the electric power supply to the heater in accordance with a detected temperature.
The heater is passed through the fixing roller or other heating member, and the both ends of the heater are supported on a support frame. In consideration of thermal expansion of the heater, as disclosed in Japanese Unexamined Patent Publication No. HEI 9-251253, one end of the heater is expandably placed in the support frame to allow a thermal expansion of the heater, and the other end of the heater is fixedly attached to the support frame by using a bracket and screws. One end of the heater is fixedly attached to the support frame to suppress the shifting of the heater in an axial direction due to a thermal expansion so that a heating region (irradiation region) of the heater can be reliably detected by the temperature sensor. However, this mechanical attachment of the heater is troublesome because the bracket must be delicately set at a given position and the screws must be rotated in a limited space.
Recently, two or more heaters are provided in a fixing roller or other heating member for the reason of raising the surface temperature of the fixing roller in a shorter time or other considerations. In the provision of a plurality of heaters, these heaters are required to be arranged as close to the axis of the fixing roller or other member as possible to ensure uniform heating of the entire surface of the fixing roller. However, the arrangement of the plurality of heaters closer to each other causes the respective ends of the heaters to become closer, consequently requiring complicated and delicate handling of the respective power transmission wires extending out of the heaters, and resulting in some difficulty in the assembling operation of a fixing device.
In the conventional way, further, the plurality of heaters are individually attached to the support frame. Accordingly, a delicate attention is required to attach the plurality of heaters keeping the heaters at a predetermined relative spatial position.
Moreover, there have been proposed a heater provided with insulators on the both ends thereof. However, it is difficult to arrange these heaters in a horizontal direction or vertical direction closer to one another because the insulator of one heater is likely to come into contact with the insulator of another heater.